1. Field of the Invention
The present invention relates to a front panel which is used for a display device of various types of devices such as a control device.
2. Description of the Related Art
To a display device, a design sheet which is composed of a thin sheet (a polyethylene terephthalate (PET) sheet) is bonded to a front face of the display device so as to protect a touch panel or a transparent plate for protecting a liquid crystal display panel (hereinafter merely referred to as touch panel) which is provided to the display device and so as to add a design to the display device. A display screen such as a liquid crystal display (LCD) can be viewed from the outside through this design sheet and the touch panel. Accordingly, a wrinkle or peel-off generated on this design sheet interferes with precise observation of the display screen. Meanwhile, when a thermal expansion rate of the design sheet and a thermal expansion rate of a member used for bonding the design sheet are different from each other, temperature change of an installation environment causes generation of a wrinkle or peel-off of the design sheet.
Japanese Patent Application Laid-Open No. 2007-293488 discloses such configuration that a metal material, a thermal expansion rate of which is equivalent to that of a design sheet, is provided on a housing of a display device and the design sheet is bonded to the metal material so as to eliminate such difference between thermal expansion rates of the design sheet and a member for bonding the design sheet.
FIG. 9 is a schematic view illustrating a front panel, according to a first prior art example, in which generation of a wrinkle or peel-off of a design sheet is prevented and which is described in the patent document mentioned above.
In this prior art front panel, a design sheet 2 protrudes to the outside more than an outer edge of a touch panel or a transparent plate for protecting a liquid crystal display panel (hereinafter, referred to merely as a touch panel) 3. This protruding portion of the design sheet 2 is bonded to a metal sheet 4, which is formed integrally with a housing 9 of a display device, with a double-sided tape 5 or the like. A liquid crystal display (LCD) 8 is disposed on a back face of the touch panel 3, and thus the front panel is configured so that an LCD display region 8a of the LCD 8 can be observed from the outside through a transparent display region A of the design sheet 2. Here, a reference numeral 19 denotes a double-sided tape or an adhesive which bonds the touch panel 3 with the housing 9.
In this front panel of FIG. 9, the design sheet 2 is not bonded to the housing 9 which is made of resin or the like but to the metal sheet 4 a thermal expansion rate of which is equivalent to that of the design sheet 2. Therefore, the design sheet 2 and the metal sheet 4 expand and contract approximately equally with respect to temperature change of an installation environment, so that generation of a wrinkle or peel-off of the design sheet 2 can be prevented.
FIG. 10 is a schematic view illustrating a front panel, according to another prior art example, in which generation of a wrinkle of a design sheet is prevented.
In this prior art front panel, a face of a touch panel 3 and a face of a design sheet 2 are configured to have identical dimensions to each other and this design sheet 2 is bonded to the touch panel 3 with an optical transparent double-sided tape 6. Further, a bottom face (a face opposite to a face on which the design sheet 2 is bonded) of the touch panel 3 is bonded to a housing 9 of a display device with a double-sided tape or an adhesive 19. Further, a liquid crystal display (LCD) 8 is disposed on a back face of the touch panel 3, and thus the front panel is configured so that an LCD display region 8a of the LCD 8 can be observed from the outside through a transparent display region A of the design sheet 2.
In this prior art example illustrated in FIG. 10, the design sheet 2 is bonded to the touch panel 3 with the optical transparent double-sided tape 6 so as to prevent generation of a wrinkle and further, deflection of the design sheet 2.
In the prior art technique, such as the technique illustrated in FIG. 9, in which a front panel is formed such that the design sheet, which protrudes to the outside more than an outer edge of a touch panel (or a transparent plate for protecting a liquid crystal display panel), is bonded to a housing or the like of a display device, the design sheet is bonded to a metal sheet of the housing to cover the touch panel after the front panel is attached to the housing and assembled. Therefore, it is difficult to align bonding end faces of the metal sheet (housing) and the design sheet and the metal sheet and the design sheet may be bonded with misalignment.
Further, a slight difference in level is generated between a surface of the metal sheet of the housing and a surface of the touch panel due to tolerance or the like. The design sheet is a thin sheet the rigidity of which is extremely low, so that the slight difference in level is directly reflected as a difference in level on a surface of the design sheet to cause degradation of quality of the display device.
Further, in order to bond the design sheet onto the touch panel with the optical transparent double-sided tape as a prior art technique such as the technique illustrated in FIG. 10, it is necessary to eliminate bubbles from a bonding face and bond the design sheet to the touch panel or the transparent plate for protecting a liquid crystal display panel with the optical transparent double-sided tape. Therefore, processing using a bonding apparatus is required. However, in the prior art method in which a unit is configured by sequentially mounting a housing, a metal sheet, and a design sheet, a bonding apparatus is not able to be used and thus, it is impossible to bond the design sheet while eliminating bubbles from a bonding face of an optical transparent double-sided tape.
Further, an optical transparent double-sided tape is made of a soft material. In a front panel, which is used in a bad environment such as in a display device of a machine which is installed in such environment that a cutting fluid scatters, for example, a cutting fluid or the like may attach on a bonding end face of an optical transparent double-sided tape. Thus, there is a concern about durability of the bonding end face of the optical transparent double-sided tape.